1. Field of the Invention
The invention relates to a molecular fluorine (F2) laser, and particularly to an F2-laser having enhanced efficiency, line selection and line narrowing of the selected line.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, followed by the next generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) lithography may use the F2-laser operating around 157 nm.
The emission of the F2-laser includes at least two characteristic lines around xcex1=157.629 nm and xcex2=157.523 nm. Each line has a natural linewidth of around 15 pm (0.015 nm). The intensity ratio between the two lines is I(xcex1)/I(xcex2) 0.7. See V. N. Ishenko, S. A. Kochubel, and A. M. Razher, Sov. Journ. QE-16, 5 (1986). FIG. 1 illustrates the two above-described closely-spaced peaks of the F2-laser spontaneous emission spectrum.
Integrated circuit device technology has entered the submicron regime, thus necessitating very fine photolithographic techniques. Line narrowing and tuning is required in KrF- and ArF-excimer laser systems due to the breadth of their natural emission spectra ( greater than 100 pm). Narrowing of the linewidth is achieved most commonly through the use of a wavelength selector consisting of one or more prisms and a diffraction grating (Littrow configuration). However, for an F2-laser operating at a wavelength of approximately 157 nm, use of a reflective diffraction grating may be unsatisfactory due to its low reflectivity and high oscillation threshold at this wavelength. In this regard, a master oscillator-power amplifier design has been proposed by two of the Applicants of the present application (see U.S. patent application Ser. No. 09/599,130, which is assigned to the same assignee as the present application and is hereby incorporated by reference) for improving the power of the output beam and enabling very narrow linewidths ( less than 1 pm), e.g., using a diffraction grating and or etalons, each preferably in combination with a beam expander. The tunability of the F2-laser has been demonstrated using a prism inside the laser resonator. See M. Kakehata, E. Hashimoto, F. Kannari, M. Obara, U. Keio Proc. of CLEO-90, 106 (1990).
F2-lasers are also characterized by relatively high intracavity losses, due to absorption and scattering in gases and all optical elements, particularly in oxygen and water vapor which absorb strongly around 157 nm. The short wavelength (157 nm) is responsible for the high absorption and scattering losses of the F2-laser, whereas the KrF-excimer laser operating at 248 nm does not experience such losses. Therefore, the advisability of taking steps to optimize resonator efficiency is recognized in the present invention. In addition, output beam characteristics are more sensitive to temperature induced variations effecting the production of smaller structures lithographically at 157 nm, than those for longer wavelength lithography such as at 248 nm.
It is therefore an object of the invention to provide a F2-laser wherein one of the plural emission lines around 157 nm is efficiently selected.
It is a further object of the invention to provide the above F2-laser with efficient means for narrowing the selected line.
Therefore, the present invention provides a F2-laser wherein one of the plural lines of its output emission spectrum around 157 nm is selected, e.g., xcex1 (see above), for its use in lithography systems. The present invention also includes, and provides means for, narrowing the linewidth of the selected line. More specifically, the present invention uses a first etalon for selecting one of the plural lines around 157 nm of the F2-laser, which also functions to narrow the selected line. Alternatively, the first etalon performs line selection and another optical element such as a second etalon narrows the selected line. Also, alternatively, another element such as a second etalon, a prism, a grating or a birefringent plate selects the line and the first etalon narrows the selected line. Also, a first etalon and a second optical element such as a second etalon, prism or prisms, a birefringent plate or a grating may be used in combination to select and narrow the primary line (i.e., xcex1). The first and/or second etalons may be used in reflective mode as a resonator reflector component or in transmissive mode disposed between the resonator reflectors of the laser system.
In a preferred arrangement, a transmissive etalon is disposed after a prism beam expander in the laser resonator. A highly reflective (HR) resonator reflector follows the etalon. Alternatively, the etalon also functions as the HR resonator reflector. Also, the prism beam expander and etalon may be on the outcoupling side of the laser chamber, wherein the etalon may be transmissive followed by a partially reflecting outcoupler mirror or the etalon may function as the outcoupler. One of the resonator reflectors, e.g., that which is also usd to outcouple the beam, may also be used to seal the laser chamber as a window on the chamber, thereby providing a more efficient laser resonator. The line-narrowing unit including the etalon and the prism beam expander is preferably in an inert gas purged module that is substantially free of species that photoabsorb radiation around 157 nm.
When two etalons are used, preferably one of the etalons is used for line selection and the other for narrowing of the selected line, or the two are used together for line selection and narrowing of the selected line. Also, one of the etalons may be used for output coupling or as a highly reflective resonator reflector, or one may be used for output coupling and the other as a highly reflective resonator reflector, such as would be configured for use in reflective mode, or one or both may be used in transmissive mode.
The gas mixture pressure and components and their concentrations are selected for improved operation of the laser, including preferably using neon as a buffer gas, either as the sole buffer gas or in combination with helium as a second buffer gas, having a total pressure less than 5 bars and a having a fluorine concentration in a range between 0.05% and 0.20%. The number of optical interfaces, i.e., optical elements, in the resonator is reduced, and the materials and other properties comprising the optical elements and laser gas are selected to provide enhanced output beam characteristics.
The etalon plates preferably comprise a material having a substantial transmissivity at 157 nm, such as CaF2, MgF2, LiF2, BaF2, SrF2, quartz and fluorine doped quartz. The etalon plates are separated by one, two or preferably three spacers comprising a material having a low thermal expansion constant, such as invar, zerodur, ultra low expansion glass, or quartz. A gas, such as helium, another inert gas such as krypton, neon, argon or nitrogen, or generally a gas that does not absorb radiation strongly at 157 nm, or a solid such as one of those mentioned above for the plates fills the gap between the etalon plates, or the gap is evacuated. The etalon may be piezo-electrically tuned, or rotationally tuned, or pressure tuned.